For many years yarns have been false twisted in order to impart various degrees of bulk and stretch properties to man-made yarns. As the speeds of machines have increased, there has been a move away from the pin twist units previously used towards friction twisting, which can insert high levels of twist at high yarn throughput speeds. However, despite the many improvements which have been made in the design of friction twist units, particularly in relation to the three disc stack type of false twist unit which is now in very common use, the current friction false twist devices are not entirely satisfactory as regards the processing of fine denier yarns at low twist levels such as are required for torque yarns as used in the manufacture of sheer hosiery. Consequently, many hosiery yarn producers still make use of pin twisters with the inherent relatively low throughput speed limitations.
Alternatively, some producers use a lower than normal temperature of the yarn heater when using a friction disc false twist unit so that not all of the high twist inserted by the friction disc unit is set in the yarn. Although this method counters the high twist level, the resulting product is limited in its applications, and process flexibility, product stability and appearance are poor.
It is known to false twist a running yarn by passing it around the surface of a roller in a helical path extending around and along the cylindrical surface thereof. The roller may be driven or may be freely rotatable so as to rotate due to the passage of the yarn therearound. False twist apparatus of this type is decribed in each of British Pat. Nos. 1280470, 1231156 and 1185684.
In the arrangement described in British Pat. No. 1280470, the roller is driven by the yarn, and the level of twist inserted in the yarn by the roller is governed by the friction characteristics of the twist inserting part of the roller and the relative speed of the yarn and the surface of that part of the roller. This latter relationship is dependent on the relative diameters of the twist inserting part of the roller and the drive receiving part around which the yarn travels in a circumferential path and not a helical path. Consequently, if differing twist levels are required for differing yarns, it is necessary to change rollers, so that a stock of rollers of differing configurations is required for each processing station of each machine.
In the case of the apparatus described in British Pat. Nos. 1231156 or 1185684, the change in the level of twist inserted in the yarn can be altered by adjustment of the angle of inclination of the roller to the general forwarding direction of the yarn path in the region of the roller.
In the case of the apparatus of British Pat. No. 1231156, the rollers are freely rotatable and each roller is mounted on a spindle which includes an adjustable knuckle joint as well as an adjustable mounting. As a consequence, it is difficult to arrange that all of the rollers in a multi-station textile machine, and from one machine to another, are set at precisely the same angle to the yarn path to ensure uniformity of processing of the yarns. Also it is a time consuming operation to set all of these rollers.
In the case of the apparatus of British Pat. No. 1185684, the inclination of the roller to the yarn path may again be adjustable in the case that the roller is freely rotatable, with the attendant disadvantages referred to in relation to the apparatus of British Pat. No. 1231156. In addition, the driving of the roller by the yarn introduces variable twisting characteristics along the length of the yarn and from yarn to yarn at each processing station of one or more textile machines. More importantly, damage to the yarn or frequent yarn breakages may be caused, particularly with fine denier yarns, by transmission of the driving force for the roller from the yarn to the roller. Alternatively, however, the roller may be positively driven by the machine drive means, and this leads to additional complications if the roller inclination is to be adjusted. To counteract this problem, it is proposed that the guide upstream of the roller is movable transversely of the roller axis so as to alter the approach angle of the yarn to the roller. However, this deflects the yarn from its natural yarn path from the heater to the roller, and this can cause irregular twist insertion and damage to the yarn in its heated state as it is deflected around the yarn guide.